java.net.UnknownHostException: broker

[mgra-zlk]

Reproduce

Note

The error could be reproduced in uc-iot/motor but probably exists also in other modules.
Similar errors with localhost instead of broker can be solved with Solution 1 below (replace broker with localhost).

1. cd uc-iot/motor
2. mvn clean spring-boot:run

Actual

The following errors occurs after the startup process

java.net.UnknownHostException: broker
	at java.base/java.net.InetAddress$CachedLookup.get(InetAddress.java:909) ~[na:na]
	at java.base/java.net.InetAddress.getAllByName0(InetAddress.java:1702) ~[na:na]
	at java.base/java.net.InetAddress.getAllByName(InetAddress.java:1582) ~[na:na]
	at org.apache.kafka.clients.DefaultHostResolver.resolve(DefaultHostResolver.java:27) ~[kafka-clients-4.1.1.jar:na]
	at org.apache.kafka.clients.ClientUtils.resolve(ClientUtils.java:125) ~[kafka-clients-4.1.1.jar:na]
	at org.apache.kafka.clients.ClusterConnectionStates$NodeConnectionState.resolveAddresses(ClusterConnectionStates.java:536) ~[kafka-clients-4.1.1.jar:na]
	at org.apache.kafka.clients.ClusterConnectionStates$NodeConnectionState.currentAddress(ClusterConnectionStates.java:511) ~[kafka-clients-4.1.1.jar:na]
	at org.apache.kafka.clients.ClusterConnectionStates.currentAddress(ClusterConnectionStates.java:173) ~[kafka-clients-4.1.1.jar:na]

Should

Messages like these should be logged:

2026-04-01T12:54:34.175+02:00  INFO 46485 --- [   scheduling-1] c.z.t.kafka.iot.motor.MotorProducer      : Sending measurement of {"sensor_id": "motor01", "datetime": 1775040874175, "value": "on"} for motor motor01 to topic plant01
2026-04-01T12:54:37.448+02:00  INFO 46485 --- [   scheduling-1] c.z.t.kafka.iot.motor.MotorProducer      : Sending measurement of {"sensor_id": "motor01", "datetime": 1775040877448, "value": "off"} for motor motor01 to topic plant01
2026-04-01T12:54:40.722+02:00  INFO 46485 --- [   scheduling-1] c.z.t.kafka.iot.motor.MotorProducer      : Sending measurement of {"sensor_id": "motor01", "datetime": 1775040880722, "value": "on"} for motor motor01 to topic plant01
2026-04-01T12:54:43.999+02:00  INFO 46485 --- [   scheduling-1] c.z.t.kafka.iot.motor.MotorProducer      : Sending measurement of {"sensor_id": "motor01", "datetime": 1775040883999, "value": "on"} for motor motor01 to topic plant01

Solutions

Solution 1

Both solutions worked on Maven 3.9.11 / Java 24.0.2, MacOS 26.4, Apple M4 Pro, Podman 5.8.1

Add the following line in /etc/hosts or C:\Windows\System32\drivers\etc\hosts, save and try again.

127.0.0.1	broker

Solution 2

Change the port of the bootstrap-server in the file uc-iot/motor/src/main/resources/application.yaml to:

spring:
  kafka:
    bootstrap-servers: localhost:29092

and try again.